The Characteristic Of Cloud Condensation Nuclei In North China Plain

Cloud condensation nuclei(CCN) have an important role in the water cycle and climate change.CCN can participate the cloud microphysical processes and determines the concentration and original particle size of the cloud droplet, then influence the precipitation efficiency. Besides that, under a certain supersaturation, high CCN concentration will decrease the droplet diameter, this may lead to a more reflective cloud (Twomey, 1977), on the other hand, the reduction of the droplet can inhibit the precipitation and extent the lifetime of clouds (Albrecht, 1989). The impact of anthropogenic emissions on aerosol and cloud properties is significant. The climate effect of cloud is extented to the local and regional scale. The cloud climate effect is one of the most factors that lead to the uncertainty in the climate prediction in the forth IPCC assessment. Therefore, the research of CCN properties will have important significance on weather and climate prediction.The CCN measurements have been carried out at WuQing and Taishan Meterological stations. The observation period was from November to December of 2009 and September to October of 2010, respectively.The CCN concentration has been measured directly using CCNC at Wuqing. The CCN concentration varies largely in winter, ranged from 4000 to 32000 cm-3 at water vapor supersaturateion of 1%. The CCN concentration had been affected by the wind speed significantly. Under the wind speed of 1.5-3.3 m s^-1, the CCN concentration was very high, and could reach 16000 cm-3 at 1% supersaturation, but the CCN concentration was just one forth of this value with the wind speed of 3.5-5.5 m s^-1; The CCN concentration varied largely in the range of 0.1% and 0.4% supersaturation. The increase of CCN concentration for every 0.1% supersaturation in the range of 0.1-0.4% is about 5 times of those in the range of 0.4% and 1.0% supersaturation. The activation rate calculated by CCN divided by CN was also affected significantly by the wind speed in the low supersaturation ( 0.1%, 0.2%), and the value under the wind speed smaller than 1.5 m s^-1 is about three times of that under the wind speed larger than 5.5 m s^-1, but at 1% supersaturation, the activation rate showed less difference with wind speed. The diurnal variation of CCN concentration showed two peaks around 8:00 and 18:00 of Beijing time, which suggests the effects of local emissions. The power law fit for the relationship of CCN concentration and supersaturation showed that the supersaturation spectrum of CCN at WuQing belongs to continental type.Size-resolved CCN efficiency spectra were measured with CCNC coupled to a Differential Mobility Analyzer and a condensation particle counter at TaiShan meteorological station. Aerosol chemical species and particle number size distribution also were measured by Aerosol mass spectrometer and SMPS simultaneously. The result shows:the max active rate of the size-resolved CCN active spectrum is close to one under the supersaturation of 0.1-0.28%,and the composition of the aerosol is much homogeneous,but the max active rate of the size-resolved CCN active rate under the supersaturation of 0.45,0.7% is about 90% with a little higher heterogeneous.The size-resolved CCN active spectrum coupled with the aerosol size distribution spectrum can give the total CCN concentration and total active rate,during the observation period, the average total concentration under the supersaturation of 0.1-0.7% is[2163±1679 cm^-3,6972±4776 cm^-3],and the total active rate is [0.26±0.15,0.79±0.19].For the heavy burning period, CCN concentration and active rate is 87% and 27% higher than the average value of the observation period, the reason is that the total aerosol concentration and the ratio of particles larger than 55nm increase significantly during the burning period, and its promotion effect is larger than the aerosol composition's inhibition effect on aerosol active.Besides that, the CCN concentration and active rate are affected by the wind direction obviously,in the supersaturation of 1%,the number concentration in the northwest is 90% higher than that in the northeast, active rate in the southwest is 18% higher than northeast; the differences above is due to aerosol number size distribution, the concentration of particles above 30nm under the northwest is highest of all and the aerosol under the southwest have more ratio large particles, the peak diameter of the aerosol size distribution spectrum is about 150nm, and under the other wind direction the peak diameter are all about 60nm.Two methods were used to derive the effective hygroscopicity parameter, (1) fit the CCN efficiency spectra with cumulative Gaussian distribution function (CDF) to obtain the critical diameter, then abtain the effective hygroscopicity parameter k_a, k_t using k-K(O|¨)hler equation. (2) calculate the effective hygroscopicity parameter k_p based on the bulk concentration of aerosol chemical composition. Both k_a and k_p have a consistent trend,but the correlation of k_a and k_p decrease with supersaturation increasing.Three models (classical power law model, modified power law model, k-K(O|¨)hler model) were used to predict the CCN concertration.The prediction of classical power law is influenced by CCN concentration fluctuation largely, its average relative deviation is about 140% under all of the 5 supersaturations.But during the burning period,the relative deviation is 56% under the supsaturation of 0.1% and it is smaller than 30% in the other supersaturation.The relative deviation using modified power law is less than 20% at high supersaturation(0.45%,0.7%), but it could exceed 170% at the supersaturation of 0.1%. For k-K(O|¨)hler model, when the measured three-hour average aerosol size distribution is used, whether the effective hygroscopicity parameter is three-hour average or ovbservation period average, the relative deviation of predicted CCN concentration is less than 20%, which suggests the closure could be reached using ?-K?hler model, but with the the average number size distribution of the entire measurement period and variable effective hygroscopicity parameter, the calculated CCN concentration could not reflect the measured CCN concentration, or reflect its variation with time. According to our measurement at TaiShan meteorological station, the aerosol number size distribution is more important than the chemical composition to predict the CCN concentration although chemistry determine the CCN activity of single particle.